1. Field of the Invention
High-purity polycrystalline silicon (polysilicon) serves as a starting material for production of monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone melting (FZ) processes, and for production of mono- or polycrystalline silicon by various pulling and casting processes for production of solar cells for photovoltaics.
2. Description of the Related Art
Polysilicon is typically produced by means of the Siemens process. This involves introducing a reaction gas comprising one or more silicon-containing components and optionally hydrogen into a reactor comprising support bodies heated by direct passage of current, silicon being deposited in solid form on the support bodies. The silicon-containing components used are preferably silane (SiH4), monochlorosilane (SiH3Cl), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), tetrachlorosilane (SiCl4) or mixtures of the substances mentioned.
The Siemens process is typically conducted in a deposition reactor (also called a “Siemens reactor”). In the most commonly used embodiment, the reactor comprises a metallic base plate and a coolable bell jar placed onto the base plate so as to form a reaction space within the bell jar. The base plate is provided with one or more gas inlet orifices and one or more offgas orifices for the departing reaction gases, and with holders which help to hold the support bodies in the reaction space and supply them with electrical current.
Each support body usually consists of two thin filament rods and a bridge which connects generally adjacent rods at their free ends. The filament rods are most commonly manufactured from mono- or polycrystalline silicon; less commonly, metals, alloys or carbon are used. The filament rods are inserted vertically into electrodes present at the reactor base, through which they are connected to the power supply. High-purity polysilicon is deposited on the heated filament rods and the horizontal bridge, as a result of which the diameter thereof increases with time. Once the desired diameter has been attained, the process is stopped.
JP 2002241120 A2 discloses a deposition reactor in which reaction gas is introduced at the top. The reaction gas mixes above the silicon rods with ascending reaction gas and then descends at the reactor wall.
In the course of this, fresh gas in a first embodiment is injected at the top of the reactor in the direction of the base plate, and in a 2nd embodiment at the upper end of the cylindrical reactor section radially from the reactor wall in horizontal direction toward the center of the reactor. Reaction gas ascending as a result of natural convection at the hot rod surface is supposed to mix with cold descending fresh gas. The descending fresh gas causes a countercurrent flow to the ascending reaction gas.
This creates a gas backup; the formation of additional gas vortexes and heating of the reaction gas, popcorn growth and/or dust deposition are the result. The specific energy demand cannot be reduced in this way.
DD 64047 A describes a process for producing polysilicon, in which the reaction gas is introduced at the top of the reaction chamber via a gas line for protection of the wall from deposits.
AT 220591 B discloses a vessel for production of high-purity silicon, in which the input gas is blown directly onto the heated silicon rod along various rod positions.
CN 201313954 Y discloses a deposition reactor in which reaction gas is injected centrally from the top, and laterally from the bottom. The gas flow generated is supposed to reduce the thickness of the gas interface layer at the silicon rod. More rapid and homogeneous silicon growth would be the result. The injection centrally from the top and laterally from the bottom described in CN201313954 Y results in a strong current toward the silicon rod bridges. A disadvantage of this process is that the opposing gas flows from the bottom and the top cancel out the gas pulses therein. This results in thicker interface layers at the silicon rods, which cause inhomogeneous and slower silicon growth on the rods.
Injection from the side (as described in AT220591 B) directly (vertically) onto the silicon rod leads inevitably to inhomogeneous rod growth and corresponding “dents” in the silicon rod.
US 2011229638 A2 describes a process for polysilicon deposition, wherein the reactor is operated with various nozzle groups which can be charged at different mass flow rates.
A standard way of producing polysilicon is to use deposition reactors in which the reaction gas is injected via nozzles in the lower section, called the base plate.
With increasing reactor diameter and increasing reactor height, it is necessary to inject correspondingly large amounts of reaction gas into the reactor with correspondingly adjusted pulse flow rate, in order to generate a sufficiently developed circulation flow in the reactor. The downward flow is at the reactor wall.
The high reaction gas and pulse flow rates required can lead to thermal stress on the silicon rods in the reactor. This is manifested by inhomogeneous rods, undesirable rod morphology (popcorn) and cracks/rod flaking. Regions with a very rough surface (“popcorn”) have to be separated at a later stage from the rest of the material, which is disadvantageous and worsens the yield.
Cracked and flaking rods can lead to electrical failure of the plant. Plant shutdowns and material wastage lead to higher production costs.
One cause of energy consumption in deposition plants is the convective release of heat via the reaction gas to the cooled reactor wall. This problem gave rise to the following objective of the invention: the reaction gas has to be introduced into the reactor in such a way that a maximum feed mass flow rate causes relatively low thermal stress on the rods. The heat loss via the wall resulting from the gas flow in the reactor is to be minimized.